The invention relates to a method of operating an internal combustion engine such as more particularly the engine of a private car.
Private car engines have a wide field of application. They have to fulfill extreme requirements. On the one hand they have to be capable of starting at low environmental temperatures while on the other hand they have to provide a substantial power output, for example when traveling on highways, at a high speed of rotation. Normally, that is to say in the case of more than 90% of their life, private car engines are however operated at low speeds of rotation and with low loads. It is appropriate to emphasize three significant operational conditions, that is to say starting up from cold, low partial loading and full loading. These three modes differ to a very great extent as regards the combustion gas temperature shortly before and during the expansion stroke so that there are differences in fuel consumption and emission of exhaust gases. In the case of diesel engine the noise is also affected to a substantial extent.
The temperature of the combustion gas, that is to say the level and the variations in temperature with time and position, of the fuel air and fuel molecules reacting with each other in the combustion chamber, and the products of reaction thereof, termed for short the combustion mixture of combustion gases, is dependent in its decisive phase shortly before and during the expansion stroke to a great extent on the instantaneous operating temperature of the engine and more particularly on the temperature of the walls of the combustion space and on the increase in the temperature of the combustion air by the compression in the engine, and furthermore it depends on the temperature of the air on entry into the combustion chamber, and the mass relationship between the combustion air and the fuel.
The engine operating temperature depends on the instantaneous load state and on the speed of rotation, as well as on the effects of the immediately preceding operational states, as for example on whether the is trend is for the temperature to increase or to decrease.
The increase in the temperature of the combustion air by the compression of the engine is primarily dependent on the compression ratio which is normally dependent on the design, but it also depends on the operating temperature of the engine. If the operating temperature of the engine is high, then the increase in temperature caused by compression in the engine will also be high, because less heat will be given up from the combustion air and the compression pressure and thus the supply of energy from the starter and due to the moment of inertia of the engine to the combustion air will be kept at a high level. If the operational temperature of the engine is low, as for example when starting from cold, the increase in temperature due to compression will also be low. The two significant components influencing the level of the compression temperature will thus mutually increase each other.
The level and variations in the combustion air temperature directly before and during the expansion stroke are the important facts having an effect on the quality of the combustion in the engine. They determine the fuel consumption and the level of noxious exhaust gas emission and the noise output in the case of a diesel engine. Furthermore the level of, and variations in, the pressure during the expansion stroke also plays a role. Within limits the temperature and pressure variations are mutually interchangeable. That is to say a drop in the pressure has an effect similar to a drop in the temperature and vice versa.
Since the stipulation of a fixed compression ratio represents a compromise for all the operational states of the engine, the optimum combustion gas temperatures are practically speaking not adhered to in any operational state at all, because an increase in temperature of the combustion air by the engine compression, as for instance when starting from cold, is too low so that the combustion air temperatures are substantially under the optimum, while at full load the temperature increase due to the engine compression is too high and for this reason the combustion air temperatures are far above the optimum. Even in the lower partial load conditions, which are so significant in duration, the optimum combustion air temperature is hardly adhered to.